1. Field of the Invention
The present invention relates to a quadrature modulated luminance/color signal separating device applied in a color television receiver, and more particularly to a spectrum distribution adaptive luminance/color signal separating device which can prevent dot crawling and blurring of color signal due to cross-color and cross-luminance.
2. Description of the Prior Art
A quadrature modulated color television system (National Television System Committee (NTSC) and Phase Alternation by Line (PAL) systems) can effectively use frequency band by interleaving a color signal into a luminance signal. However, if luminance and color signals are falsely separated in a receiver, deterioration of picture quality such as dot crawling, blurring of color signal, or reduction of resolution is generated.
To solve the above problem, a line comb filter and a frame comb filter using the characteristic that phase of color signal is inverted by 180.degree. every line and every frame in NTSC signal are suggested. Also, a motion adaptive luminance/color signal separating device which adaptively uses these filters in an image motion is suggested.
FIG. 1 shows a spectrum distribution of a general NTSC color television signal. The bandwidth of luminance signal Y is 4.2MHz, and in a color signal, I(In-Phase) bandwidth is 1.2MHz, and Q(Quadrature-Phase) bandwidth is 0.5MHz.
FIG. 2 shows a spectrum distribution of luminance signal and color signal near a color subcarrier. The spectrum of luminance signal is repeatedly shown with respect to n.cndot.Fn (n: integer, Fn: line frequency .apprxeq.5.75KHz), and the spectrum of color signal is repeatedly shown with respect to (2n+1)xFn/2. Meanwhile, an image having a large change in a vertical direction shows that the spectra of luminance and color signals are severely interfered.
Referring to FIG. 3, a generally used 2-line or 2-frame delay type comb filter comprises a band-pass filter 1 for band-passing a composite video signal CV with respect to a color subcarrier fsc at the center, delay elements 2A and 2B for delaying an output signal of the band-pass filter 1 for a predetermined time (one line or one frame), 1/2 amplifiers 3A and 3B for 1/2-amplifying the respective output signals of the band pass filter 1 and the delay element 2B, an adder 4A for detecting a color signal C by subtracting the respective output signals of the amplifiers 3A and 3B from an output signal of the delay element 2A, a 1/2 amplifier 3C for 1/2-amplifying a color signal outputted from the adder 4A and outputting it as a final color signal C, and an adder 4B for outputting a luminance signal Y by subtracting an output signal of the 1/2 amplifier 3C from the composite video signal CV.
Also, referring to FIG. 4, a generally used motion adaptive luminance/color signal separating device comprises a line comb filter 5 and a frame comb filter 6 for respectively separating a line signal and a frame signal from a composite video signal CV, a motion detecting section 7 for determining whether the inputted composite video signal CV is in a motion region, a mixer 8 for respectively selecting a line comb filtered signal when the signal determined in the motion detecting section 7 is in the motion region, and a frame comb filtered signal when it is in a still region, and an adder 9 for obtaining a luminance signal Y by subtracting a color signal C outputted from the mixer 8 from the composite video signal CV.
Referring again to FIG. 3, the circuit is a 2-line comb filter if the delay period of delay elements 2A and 2B having a constant delay period is a 1-line, and is a 2-frame comb filter if it is a 1-frame. The composite video signal V1 passing the band pass filter 1 for filtering with respect to a color subcarrier fsc and the output video signals V2 and V3 of the delay elements 2A and 2B are combined in the adder 4A, from which a color signal (C=V2/2-(V1+V3)/4) is outputted. Also, the adder 4B obtains a luminance signal Y by subtracting a color signal outputted from the amplifier 3C from the inputted composite video signal CV.
Referring again to FIG. 4, the inputted composite video signal CV is simultaneously separated into a luminance signal Y and a color signal C by a line comb filter 5 and a frame comb filter 6. The separated luminance signal Y and color signal C are inputted to the mixer 8. The motion detecting section 7 determines the existence or absence of motion of video signal and outputs a control signal according to its determination to the mixer 8. According to the inputted control signal, the mixer 8 selects and outputs a luminance signal Y and a color signal C separated by the line comb filter 5 when a motion exists in an input signal, and only a color signal C separated by the frame comb filter 6 when there is no motion. Meanwhile, the adder 9 outputs a luminance signal Y by subtracting a color signal C outputted from the mixer 8 from the composite video signal CV.
However, such a conventional device provides a relatively good function, but in a video signal changed in a vertical direction, reduction of vertical resolution, hanging dot, and cross-color are generated in the separated luminance/color signal. Also, the frame comb filter provides a relatively good function in an image having no change in a temporal direction, but shows deterioration of picture quality such as blurring in an image having a change in a temporal direction. Particularly, in an image having a large change in the temporal direction, separation into luminance and color signals is not performed, thereby creating the problem of badly deteriorating the picture quality. The most widely used notch filter has the problem of showing reduction of horizontal resolution, dot crawling, and cross-color. As described above, in a motion adaptive luminance/color signal separating device, since the motion detection from the composite video signal in which the luminance signal and the color signal are mixed is inaccurate, the separation into luminance and color signals is also inaccurate, thereby causing deterioration of picture quality. And, even if the detection of motion is accurately done, there is a problem in that the separation into luminance and color signals is imperfectly done when the change of image in the vertical and temporal directions is large.